In my Arneson U.S. Pat. No. 5,667,415, there is disclosed a surface piercing propeller enclosed within a metal shroud. The shroud extends over the top of the surface piercing propeller in all embodiments illustrated.
In Arneson U.S. Pat. No. 5,667,415, the water churned upwardly by the rotation of the propeller is deflected by the overlying shroud. The interaction of the overlying shroud with the blade tends to reduce the turbulence overlying the propeller. The instabilities of the boat arising from stern lift and bow immersion of the outdrive propeller are substantially reduced. Moreover, the operator finds it much easier to operate the controls of the boat since the overlying shroud acts as a partial barrier for lateral movements of the water which tend to cause the propeller to “walk” to one side of the vessel, exerting a turning force on the boat relative to the water.
The elimination of the instabilities associated with the shroud thereon clearly utilizes the positions of the inner surfaces of the shroud. The shroud is typically far enough away from the plane of rotation of propeller so as to prevent interference by the shroud to the rotation of the propeller itself as well as the shroud being drawn into the propeller. The inner surfaces of the shroud members also contribute to keeping the center shaft thrust direction stable so that there is reduced tendency for the propeller to lift out of the water and cause the operator of the boat to fight the steering and trim gears of the boat. The propeller configuration is different from standard propeller units. The propeller is smaller in diameter with wide thick blade tips that make it very strong and efficient. This allows the boat to get on plane quickly and with ease and maintains the achieved plane even when the rpms of the system are decreased (conventional boats tend to fall off plane when this occurs).
Discovery
I routinely have conducted extensive testing of outdrives in San Francisco Bay and elsewhere. As a result of this extensive testing, and through careful examination of a number test models—exceeding 100 in the last 5 years, I have made several important discoveries. The reader will understand that discovery can constitute invention by itself. More often, discoveries lead to the definition of problems to be solved. Once the problems are identified, further work can lead to the solution of those problems. Accordingly, I claim invention relative to the following discoveries, identification of problems, as well as to the solution to those problems.
First, I have discovered that the propeller characteristics of an outdrive propeller proceeding through the water at high speed are surprising and not obvious, even after thousands of hours of testing. In order to understand these discoveries, it is necessary to review the fundamentals of out drives.
A typical out drive trails the transom of a high speed planing hull. The outdrive propeller is typically immersed below the surface of the water from the center of rotation of the propeller to immerse just the lower half of the propeller within the water, presuming that the water is undisturbed. The shaft of the propeller extends from the transom downward at an angle with respect to the surface of the undisturbed water when the high speed planing hull is on plane. This has the beneficial result of keeping the most of the shaft of the outdrive out of the water. Typically, this angle can be from 6° to 12°. I will use 6° in the following examples.
The shaft of typical outdrive is typically of large diameter. It includes an outer tubular housing and an inner rotating shaft to supply rotational power to the propeller. Typically, the driving shaft is supplied with two sets of bearings. A first bearing adjacent is a universal joint on the shaft with the universal joint enabling the shaft to be “steered.” The second bearing is immediate the propeller at the distal end of the shaft from the boat. Having the shaft extend from the transom of the boat, downward at an angle of 6° to 12° from the horizontal, the major part of the shaft and surrounding tubular member is kept from having to be dragged through the water. This saves considerable friction with respect to the water and this angular disposition of outdrives is universally used.
In the following description, I am going to use the definition “working surface” to describe an arbitrarily selected portion of a propeller blade. I will select this arbitrary “working surface” by measuring radially outward of the blade of a propeller, here a 14 inch diameter propeller. The radial distance that I will choose is 5 inches. I will take measurement of the angle of the working surface tangent to the rotation of the propeller.
The reader will understand the reason for this arbitrary definition. Specifically, propeller blades have changing working blade angles from the hub of the propeller to the extremity of the blades. In the usual case, the pitch is high adjacent the hub and gradually decreases as that pitch is measured radially outward. By having a “working surface” (pitch chosen on an arbitrary radial tangent to the direction of propeller rotation), it is possible to generate a convenient working definition of propeller pitch in angle with respect to the shaft. Using this definition, some of the working principles of this invention can be more easily understood.
I have discovered that the 6° downward disposition of the outdrive shaft has the effect of producing variable pitch propeller blading on opposite sides of the partially immersed propeller! Specifically, this may be seen by taking a representative “working surface” on the surface of a propeller. Say on a 14 inch diameter propeller, this chosen “working surface” happens to be in the middle of a propeller blade at a distance of 5 inches of radius from the center of rotation of a propeller having a 7 inch radius (or 14 inch diameter). Placing a level device along the “working surface” tangent to the direction of propeller rotation and measuring the angle of the “working surface” with respect to the outdrive shaft will yield a constant angle of the working surface with respect to the shaft. Say for example this angle is 54°. So at any position of rotation of the “working surface” with respect to the shaft, this angle will always be the same, that is 54° with respect to a plane including the axis of the drive shaft of the propeller.
But everyone forgets that the propeller shaft itself is at an angle! Say that angle is 6° with respect to the horizontal when the boat is planing at high speed. I have discovered that this produces variable propeller pitch on opposite horizontal sides of the propeller! As these variable propeller pitches are integral to the shrouding that I place around my improved outdrive, the variable pitches must be understood.
As is well known, most single propellers rotate counterclockwise following the well known “right hand rule.” By extending the right hand thumb in the direction of the propeller shaft, the fingers when naturally curled give the direction of rotation of the propeller. Where two propellers are used, one propeller rotates counterclockwise and the other propeller clockwise. And since both type of propellers are always a possibility in an outdrive propeller, I choose to talk about the working surfaces of the propeller entering the water and the working surfaces of the propeller leaving the water, regardless of whether the propeller right or left hand rotation.
As will be shortly developed, the entry pitch of the working surface (angle of attack with respect to the passing undisturbed water) is increased upon entry into the water by the angle of the shaft with respect to the water. Similarly, the departure pitch of the working surface is decreased upon departure from the water by the angle of the shaft with respect to the water. This discovery is an important consideration in the design that follows.
Consider the case of the entry pitch of the working surface. As we have previously developed, the working surface has a 54° angle with respect to the propeller shaft. But the propeller shaft is inclined at 6°. Adding this 6° to 54°, the angle of attack of the entry pitch of the working surface with respect to the undisturbed water though which the propeller passes upon entry into the undisturbed water level now becomes 60°!
Consider the case of the departure pitch of the working surface. Again the working surface has a 54° angle with respect to the propeller shaft. But the propeller shaft is inclined at 6°. Subtracting this 6° from 54°, the angle of attack of the entry pitch of the working surface with respect to the undisturbed water through which the propeller blade passes upon departure from the undisturbed water level now becomes 48°!
The important thing to understand, is that with an outdrive having shaft inclined from the horizontal by a small angle (here 6°), the entry pitch of any working surface on a blade is higher that the departure pitch of any working surface on the blade by the value of the shaft inclination.
Now let us talk about propeller “pitch” in general.
Where one wants rapid acceleration and high propeller output power, low pitches on propellers are desirable. For example, tug boat propellers have low pitch so that large vessels may be slowly moved. Similarly, sail boat auxiliary propellers have low pitch so that the boats may maneuver in adverse weather conditions (i.e. keeping off the rocks in heavy weather). Low pitch propellers are not intended for high speed.
Where one wants high speed, high pitches are desirable. For example, racing boat propellers have high pitch so that the racing boat can proceed at high speed. High pitch propellers are not intended for low speed.
Now let us talk about the practical effect of the pitch change in the partially immersed outdrive propeller. The entry half of the propeller has higher pitch than the departure half the propeller! So at low speed and upon acceleration, the departure pitch will be more ideal. Upon reaching higher speed, the entry pitch of the propeller will be more ideal.
It will be understood that the propellers I use in this disclosed outdrive rotate at high power and high speed; for example all of the applicable testing for this invention has been accomplished in a twin hull boat having a 4000 Hp Lycoming Gas Turbine Engine with propeller rotating speeds of 6,000 to 7,000 rpms. Propellers having mechanically variable pitches are not practicable.
Again, the reader should not confuse my testing of this outdrive with those minimal conditions necessary to make the outdrive operable. As I have emphasized, any planing hull proceeding at more than 18 mph will suffice. Further, power expended to do this can be relatively minimal. All that is needed is sufficient power to make the boat hull plane.
Having discussed my discovery of the variable pitch of an outdrive propeller, discussion of my discoveries about the disturbance of water by a propeller proceeding through the water at high speed now become relevant. In summary, I have discovered that where a boat is proceeding at high speed—say 160 mph, standing water is disturbed before the blade of the propeller passes through the standing water. In other words, there is a disturbance in advance of blade entry to the surface of the water! There is a well known disturbance after the blade passes through the water; any person standing at the stern of a propeller driven vessel and observing its wake recognizes this disturbance. It is not well known that disturbance occurs in the direction of boat travel in advance of the passage of the propeller blades through the water!
First, it may well be that shock wave transmit in water faster than the high speed (e.g. 160 mph) passage of the boat.
Second, the variable pitch phenomena related to outdrives also has an effect. Consider the following.
If a propeller is pulled through the water without rotation, the “windage” of the propeller will cause the propeller to rotate. This is a well known phenomena for sailors repairing large engines at sea on ships underway. Specifically, the shaft of the engine being repaired must be locked, and ship moved at slow speed to maintain steerage, otherwise the windage of the propeller will cause the engine under repair to rotate, creating an extraordinarily dangerous condition.
Now consider the case where the propeller is rotated at a speed which is “neutral” to the rate of the passing water. Other than displacement effects, the propeller will neither have windage nor a propulsive force.
In the usual case, the propeller is rotated to propel water at a considerably faster speed than actual passage of the boat through the water. The propeller has slippage with respect to the passing water that is essential to its propelling effect. Anyone who has observed the wake of a propeller propelled ship is familiar with this result.
Now consider the case of the outdrive of this invention. The entry side of the propeller has a higher pitch, driving the water at higher speed. The departure side of the propeller has lower pitch, driving the water at lower speed. In actual practical effect, both pitches will considerably exceed the rate of passage of the boat through the water. For example, where the boat is proceeding through the water at 160 mph, both the entry high pitch side of the propeller and the departure low pitch side of the propeller will drive water at speeds exceeding the 160 mile per hour speed of the boat.
But there will be another surprising effect. When the entry side of the propeller is compared to the departure side of the propeller; water build up in advance of the departure side of the propeller will be more pronounced than water build up in advance of the entry side of the propeller!
The reason for this water build up differential is directly related to the variable pitch between the departure and entry sides of the propeller. Specifically, since the departure side has lower pitch and moves water at the propeller more slowly, water buildup in advance of the departure of the partially immersed propeller blade will be greater. Similarly, since the entry side has higher pitch and moves water at the propeller more quickly, water buildup in advance of the entry of the partially immersed propeller blade will be lesser. As will hereafter be understood, I use the greater buildup of water on the departure side of the propeller to advantage. Specifically, I place a horizontal barrier at approximately two thirds (⅔) of the propeller radius directly overlying the departure side of the partially immersed propeller. This has the effect of keeping the low pitch departure side of the propeller immersed in water for more efficient propulsion.
Plates overlying propellers used in the prior art are known. So-called “cavitation” plates are an example. These plates, used for example over outboard propellers, prevent water “flashing” into steam (cavitation). As distinguished from my plate, these plates are over an entirely immersed propeller. In what follows, I show plates under to the top portion of the partially immersed propeller.
Further, I have used plates on outdrives on shrouds or fins, these plates being over the upper two thirds (⅔) of a propeller. However, these plates have been parallel to the shaft, and never parallel to the plane of the undisturbed water. These plates have the effect of directing reverse water jets at and over the transom of the boat to which they are attached, especially during coming up to speed or decelerating from speed.
Further, the plates have been separated by several inches (in the order of three to four [3 to 4] inches) in advance of the propeller. Plates with this spacing cannot cooperate with the accumulation of water in advance of the departure side of the propeller. Water in the gap between the propeller and plate is not controlled and cannot provide the improved propulsion of this disclosure.
I have further discovered that inversion of the shroud from the preferred embodiments shown in my Arneson U.S. Pat. No. 5,667,415 produces superior results. Specifically, I use an inverted or “upside down” shroud. The inverted shroud defines an enclosed operating channel for the surface piercing portion of the propeller which isolates the partially immersed propeller from imparting unwanted torques to high speed hulls driven by the disclosed outdrive. Stern uplift with bow immersion is avoided. Further, crawling or “helm” exerted to one or the other side of the boat is substantially reduced.
The “upside down” shroud renders the direction of propeller rotation essentially irrelevant as it forms a separate and isolated chamber from the remainder of the water that the boat is passing through. For example, whether a so-called “right hand propeller” or a “left hand propeller” is utilized is irrelevant. Further, the slope of the wake where propeller immersion occurs is not as important. The disclosed shroud has the effect of isolating what might otherwise undesired torques on the vessel propelled by my outdrive.